This invention relates to a method for preparing high impact graft polymers, based on graft copolymers of one or more monomers on elastomers. More specifically, it concerns an improved method for the preparation of rubber-modified styrene polymers having a broad distribution of rubber particle size by a two step bulk polymerization followed by a suspension polymerization in aqueous medium.
It is known to prepare rubber-modified styrene polymers by a bulk-suspension process. In this process an elastomer is dissolved in styrene monomer, polymerized in bulk to 10-50 percent conversion, suspended in aqueous medium, and polymerized in suspension to complete conversion.
Among the many disadvantages of the above two step process are the high viscosity developed during the bulk polymerization step and the generally narrow particle size range of the rubber particles formed. Material with broad rubber particle size distribution has been made successfully by blending small particle size material with large particle size material. However, the blending technique has many drawbacks. In the first place it requires the preparation of two different feedstocks which need close monitoring. Also, whenever two different materials are blended together, there is always the possibility of incompatibility, poor mixing, wrong feed ratios, etc.